1. Field of the Invention
This invention relates generally to a heatable transparency, such as a laminated vehicle windshield and, more particularly, to a heatable transparency having improved bus bar and safety features.
2. Technical Considerations
It has been known to pass electric current through a transparent, electrically conductive coating on a laminated vehicle windshield in order to raise the temperature of the windshield. Typically, a source of electrical potential is connected to the conductive coating through a pair of spaced bus bars positioned along opposite sides of the conductive coating. The bus bars are in electrical contact with the conductive coating in order to provide current flow through the coating between the bus bars. The bus bars have relatively lower resistivity compared to the conductive coating and distribute the current over the coating area to be heated. Conventional bus bars are typically formed by metallic foil strips or strips of metallic-ceramic frit material fused onto a surface of one of the sheets of the laminate. A conventional arrangement includes bus bars configured as substantially parallel strips on opposites sides of the conductive coating, with electrical leads attached to each bus bar.
Heatable windshields can also include a “cut-out” area along an edge of the windshield that is free of the conductive coating. This coating-free area permits the passage of electromagnetic energy, such as radio waves, through the windshield for uses such as an electronic toll collection (ETC) system. Conventional bus bar designs require the bus bar to bend around this coating free area to maintain contact with the conductive coating.
A crack in a heatable windshield or the conductive coating can alter the electric heating circuit in ways that can damage the transparency or the conductive coating or have other undesirable effects. A break in the conductive coating can increase the resistance in the coated areas, with the result that the power output increases in the affected area. The increased power can raise temperatures to such an extent that the transparency can be thermally damaged. Excessive temperatures can extend propagation of a crack in the glass or can melt the plastic interlayer of the windshield.
Therefore, it would be advantageous to provide a transparency, such as a laminated automotive transparency, having an improved bus bar design that allows for easier and less costly manufacture as well as improving the electrical connection between the bus bars and conductive coating. It would also be advantageous to provide a laminated transparency that provides an arrangement for detecting a crack in the transparency and/or the conductive coating.